This invention relates to transparent, high-temperature-resistant, aluminum-silicon-oxide monolithic members or coatings and, more particularly, to such members or coatings which are prepared by reacting precursors, derived from alkoxides and gelling the reacted precursors, with the gel dried in the desired configuration, and then heated to eliminate porosity.
Silica and alumina are important constituents of many of the technological products of the ceramic and glass industry. Silica is processed in vitreous form and has widespread industrial application because of its refractory nature, chemical stability, and low thermal expansion. One weakness of vitreous silica is that prolonged use above 1000.degree. C. normally caused it to transform into cristobalite, resulting in a large dimensional change which destroys the mechanical integrity of the material upon thermal cycling.
Alumina has many technical and industrial uses and occurs in a number of forms. Alumina is industrially produced in transparent form as single crystal sapphire and in sintered polycrystalline form and is extensively used as arc tubes for high-pressure sodium lamps.
Mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) is an important refractory material which is formed in the alumina-silica binary. Most of the studies of the alumina-silica binary system have involved preparation of materials by high-temperature reactions. Since both silica and alumina are relative refractory oxides, crystalline transformations take place in either oxide before substantial reaction or diffusion takes between the oxides.
In The American Ceramic Society Bulletin, Volume 54, No. 3, March, 1975, pages 286-288, article by B. E. Yoldas, is reported an active alumina which can be produced in transparent bulk form at temperatures as low as 500.degree. C. This alumina has extremely small pores which do not interact with light. Thus the material remains transparent up to 1200.degree. C., where it non-destructively converts to alpha-alumina and loses its transparency.
In Journal of Materials Science 11 (1976), pages 465-470, article by B. E. Yoldas, is reported the stabilization of active alumina through incorporation of 6% by weight silica into the structure, which increases the alpha-alumina transformation temperature from 1200.degree. to about 1380.degree. C.
The preparation of alumina sols from aluminum alkoxides is disclosed in American Ceramic Society Bulletin, Volume 54, No. 3, pages 298-290, March, 1975, article by B. E. Yoldas, and a patent corresponding to this is U.S. Pat. No. 3,941,719 dated Mar. 2, 1976.
The preparation of glasses and ceramics from metal-organic compounds is disclosed in Journal of Materials Science 12 (1977), pages 1203-1208, article by B. E. Yoldas. The reaction of ethoxysilanol with aluminum alkoxide is disclosed at page 1206 of this article. This includes forming a gel from the reacted alkoxides, drying and then firing the dried material. Such a technique can be used to form a monolithic ceramic member.